1. Field of the Invention
This invention relates to measurement of material properties, and particularly to a versatile probe for measuring electrical impedance of materials.
2. Description of Background
The composition of materials used in electronic devices is determined for a variety of reasons. For example, a European Union directive, “Restriction of Hazardous Substances” (RoHS), bans the placing on the EU market of new electrical and electronic equipment containing more than agreed levels of lead, cadmium, mercury, hexavalent chromium, polybrominated biphenyl (PBB) and polybrominated diphenyl ether (PBDE) flame retardants, and material composition is determined to ensure compliance with RoHS. One method of determining the material composition is by measuring the electrical impedance of the material. Traditionally, such impedance measurements have been accomplished through the use of a single-point or surface probe resulting in an impedance value of the material measured in ohms/square.
A measurement is performed by placing the single-point or surface probe on the material. The measurement can be influenced by an amount of force applied to the probe when placing it on the surface. However, the measured impedance can be erroneously high or low if the applied force is not sufficiently controlled. Furthermore, when taking several measurements on a material sample, the applied force must be consistent to ensure the measured impedance values are comparable. In a traditional probe, a nickel plated gasket surrounds the probe and is intended to control the amount of force applied to the probe. The gasket, however, wears or changes characteristics over time requiring its replacement. Additionally, the gasket often deforms to a compressed state over time which can influence the amount of force applied to the probe, thereby changing the impedance measurements.
Finally, the standard probe is limited to talking one particular type of measurement (e.g. ohms/square) and only one measurement at a time. To measure more than one location on a sample, the probe must be moved. To take measurements requiring more than one interface to the material sample, more than one probe or some additional devices must be used in addition to the standard probe.
What is needed is a measurement probe that can accurately control the force applied to the probe during measurement. In addition, a probe is needed that is versatile, allowing for many measurements to be performed without moving the probe, and allowing for differing types of measurements to be performed without the use of additional probes or other ancillary equipment to facilitate the measurement.